This invention relates generally to heat shrinkable thermoplastic packaging film which can be heat sealed to itself to form a flexible package. The invention relates more particularly to multilayer tubular film of ethylene vinyl acetate copolymeric material having a sealing layer that is not adversely effected during irradiative treatment.
A heat shrinkable, tubular packaging film which has enjoyed considerable commercial success for making bags having low gas transmission is described in U.S. Pat. No. 3,741,253 issued June 26, 1973 to Brax et al. One preferred film in the Brax patent may be produced by extruding a tubular substrate layer of ethylene vinyl acetate copolymer and then flatting the tube and moving it in a series of passes through an electron beam to cross-link the molecules of the copolymer to facilitate orienting. Next, the film is inflated, but not substantially stretched, and passed through an annular coating die where it receives a second or barrier layer which comprises saran being a copolymer of vinylidene chloride-vinyl chloride. Irradiation must be conducted prior to the application of the saran barrier layer since irradiation renders saran unacceptable for use in food packaging applications. This two layer tube then passes through another annular die in which it receives a coating of ethylene vinyl acetate copolymer. The tube at that point having a wall of three polymeric layers is cooled, collapsed, and fed through a hot water bath where the tube is sufficiently softened so that it can be inflated into a bubble and stretched while engaging between two spaced apart pairs of pinch rollers. The inflation causes the tubing to be thinned as it is stretched into film, and as the bubble leaves the hot water bath it cools quickly in air and becomes oriented. After the bubble is collapsed, the film is wound up as flattened, seamless, tubular film to be later used to make bags, e.g. either end-seal bags which are typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, or side-seal bags in which the transverse seals form the sides and one edge of the collapsed tubing forms the bottom of the bags.
In making bags in a rapid commercial operation wherein the flattened tubing walls are pressed together with heated seal bars, the dwell time of the seal bars should be as short as possible yet sufficiently long to fuse and bond the inside surface layer of the tube to itself in a smooth, continuous, strong seal. At rapid speeds the seals do not have time to set-up, and the tension when transporting the tubing can cause the seals to pull apart. Intermittent motion is necessary as bags are moved to and from the seal station as there must be a time when the tubing is stopped while the transverse seals are made. At a high bag making rate if the seals are not strong, the force due to the acceleration of the leading bag to advance the next bag into the seal station will cause the newly formed seals to be pulled apart as the seals are still warm. Conventional practice provides for adjustment of bag making rate to allow the seal to cool sufficiently. It is an object of the present invention to provide a film which will have strong seals at relatively high bag making rates.
The importance of a strong reliable seal further applies during use of such bags. This type of bag is primarily used to package a large variety of fresh and processed meats typically by placing the meat in the bag, evacuating the bag, gathering and applying a metal clip around the gathered mouth of the bag to hermetically seal it, and then immersing the bag in a hot water bath at approximately the same temperature at which the film was stretch-oriented. This temperature is typically in the range of 160.degree. to 205.degree. F. Hot water immersion is one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly. Thus, it is essential that heat seal integrity be maintained during shrinkage of the bag at elevated temperatures.
Attempts to enhance the seal strength of bags made from tubular film similar to the foregoing exemplary three ply film have been aimed at adding an inside layer of thermoplastic material having sealing properties superior to that of ethylene vinyl acetate copolymer. However, a major problem that has been encountered in coextruding an inside sealing layer along with the ethylene vinyl acetate copolymer tubular substrate is that upon irradiatively cross-linking the substrate, the sealing layer is either cross-linked which elevates its softening point or degraded thereby diminishing its sealability. Thus, it is an object of the present invention to provide a sealing layer having sealing properties substantially uneffected during irradiative cross-linking treatment of an associated shrink layer.
Another problem encountered in finding an acceptable sealing layer for films such as the foregoing exemplary film relates to orientation of the film. It is commercially advantageous to orient film at temperatures near that of boiling water. A conflict is posed in identifing a sealing material capable of forming a relatively high temperature high strength seal but that does not unduly hamper orientation of ethylene vinyl acetate copolymer film at temperatures near that of boiling water. Thus, it is a further object of the invention to provide a high strength seaing layer for ethylene vinyl acetate copolymer multiply film but that permits development of shrink potential comparable to that of the foregoing prior art film at an orientation temperature near that of boiling water.
Of General interest is the disclosure of U.S. Pat. No. 4,207,363 for "Flexible Heat Shrinkable Multilayer Film for Packaging Primal Meat" issued June 10, 1980 to Lustig et al, directed to a multilayer film having a first outer layer of a blend of a propylene ethylene copolymer, a (butene-1)-ethylene copolymer, and a thermoplastic elastomer selected from the group consisting of ethylene-propylene copolymers and ethylene propylene diene terpolymers; a first core layer capable of being stretched during biaxial orientation of the multilayer film and comprising an extrudable adhesive; a second core layer serving as an oxygen barrier and being compatible to the biaxial orienting and heat shrinking of the film; and a second outer layer of an ethylene vinyl acetate copolymer; the multilayer film being biaxially oriented.
Of general interest is the disclosure of U.S. Pat. No. 4,276,330 for "Trilaminate Film For Forming Sideweld Bags" issued June 30, 1981 to Stanley et al, directed to sideweld bags made from trilaminate film having a core layer of ethylene-propylene block copolymer between surface layers of polypropylene or an ethylene-propylene random copolymer.